Bio - Circadian rhythms and Ultradian & infradian rhythms Flashcards
What is a circadian rhythm?
A pattern of behaviour that occurs or recurs approximately every 24 hours, and which is set and reset by environmental light levels.
Sleep-wake cycle
Refers to alternating states of sleep and waking that are dependent on the 24-hour circadian cycle.
What are examples of circadian rhythms?
Sleep-wake cycle
Body temperature
Blood pressure
Hormone production
How long is a circadian rhythm?
24h
What type of rhythm is the sleep-wake cycle?
Circadian.
What type of rhythm is body temperature?
Circadian.
What type of rhythm is blood pressure?
Circadian.
What type of rhythm is hormone production?
Circadian.
How do circadian rhythms operate?
They are driven by our body clocks, found in all of the cells of the body, and synchronised by the master circadian pacemaker, the suprachiasmatic nuclei (SCN), found in the hypothalamus.
Explain how the sleep-wake cycle works
Light and darkness are the external signals that determine when we feel the need to sleep and when to wake up. The circadian rhythm also idps and rises at different times of the day, so our strongest sleep drive usually occurs in two ‘dips’, between 2-4am and between 1-3pm (the ‘post-lunch dip’).
The sleepiness we experience during the circadian dips is less intense if we have had sufficient sleep, and more intense when we are sleep deprived.
Not only is this cycle determined by the circadian rhythm alone, but is also under homeostatic control. So when we’ve been awake for a long time, homeostasis tells us that the need for sleep is increasing because of the amount of energy used up during wakefulness. This homeostatic drive for sleep increases gradually throughout the day and is highest in the late evening when we fall asleep.
Circadian rhythm keeps us awake as long as there is daylight and prompts us to sleep when it gets dark, and the homeostatic system tends to make us sleepier as time goes on throughout the waking period regardless of the time of day.
The internal circadian ‘clock’ is ‘free-running’ so will maintain a cycle of about 24-25h even in the absence of external cues. The circadian system is, however, intolerant of any major alterations in sleep and wake schedules (e.g. shift work or jet lag), because this causes the biological clock (and the internal physiological systems that are dependent on this) to become completely out of balance.
What is the difference between the effects of the circadian rhythm and the homeostatic system in the sleep-wake cycle?
Circadian rhythm keeps us awake as long as there is daylight and prompts us to sleep when it gets dark, and the homeostatic system tends to make us sleepier as time goes on throughout the waking period regardless of the time of day.
Why is core body temperature a circadian rhythm?
It is at its lowest (about 36 degrees) at about 4:30 am and at its highest (about 38 degrees) at about 6pm.
Normally, sleep occurs when the core temperature begins to drop, and body temperature starts to rise during the last hours of sleep, promoting a feeling of alertness in the morning. A small drop in body temperature also occurs in most people between 2pm and 4pm, which may explain why many people feel sleepy in the early afternoon.
Why is hormone production a circadian rhythm?
For example, the production and release of melatonin from the pineal gland in the brain follows a circadian rhythm, with peak levels occurring during the hours of darkness. By activating chemical receptors in the brain, melatonin encourages feelings of sleep. When it is dark, more melatonin is produced, and when it is light again, the production of melatonin drops and the person wakes.
Describe the case study for circadian rhythms
Series of studies conducted by the French cave explorer Michel Siffre. On several occasions Siffre has subjected himself to long periods of time living underground in order to study his own circadian rhythms.
While living underground he had no external cues to guide his rhythms - no daylight, no clocks or radio. He just simply woke, ate and slept when he felt it was appropriate to do so. The only thing influencing his behaviour was his internal body clock (i.e. ‘his free-running’ circadian rhythms).
After his first underground stay of 61 days in the southern Alps in 1962, he resurfaced on 17 September believing the date was really 20 August. On the second occasion, he spent 6 months in a cave in Texas. His natural circadian rhythm settled down to just over 24h but with some dramatic variations.
On his final underground stay in 1999, he was interested in the effects of ageing on circadian rhythms (he was 60 years old now). He found that his body clock ticked more slowly compared to when he was a young man, sometimes stretching his circadian rhythm to 48h.
Briefly show the pathway that the sleep-wake cycle takes
Sunlight–>Photoreceptor (retina)–>SCN (hypothalamus)
and then to either:
- (Early morning) Intense/blue light–>Cortisol (pituitary)–>Wake.
- (Sunset) Faint/red light–>melatonin (pineal)–>Sleep.
Where is the control centre for circadian rhythms?
Hypothalamus.
Infradian rhythms
Rhythms that have a duration of over 24h, and may be weekly, monthly or even annually.
Ultradian rhythms
Cycles that last less than 24h, such as the cycle of sleep stages that occur throughout the night.
How long do ultradian rhythms last?
Less than 1 day/24h.
What are examples of ultradian rhythms?
The sleep cycle/dream cycle/sleep stages.
The Basic Rest Activity Cycle (BRAC).
The glymphatic system.
Why/how are the sleep stages an example of an ultradian rhythm?
Because they follow a pattern of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep, which consists of stages one through to four.
The cycle repeats itself about every 90-100 minutes throughout the night, with different stages having different durations.
How do the sleep stages progress as an ultradian rhythm?
As a person enters deep sleep, their brainwaves slow and their breathing and HR decreases. During the 5ht stage (REM sleep), the EEG pattern resembles that of an awake person, and it is in this stage that most dreaming occurs.
Stage 1: 4-5% (light sleep, muscle activity slows down, occasional muscle twitching).
Stage 2: 45-55% (breathing pattern and HR slows, slight decrease in body temperature).
Stage 3: 4-6% (deep sleep begins, brain begins to generate slow delta waves).
Stage 4: 12-15% (very deep sleep, rhythmic breathing, limited muscle activity, brain produces delta waves.
Stage 5: 20-25% (rapid eye movement, brainwaves speed up and dreaming occurs, muscles relax and HR increases, breathing is rapid and shallow).
Why/how is the Basic Rest Activity Cycle (BRAC) an example of an ultradian rhythm?
Kleitman (1969) - 90 minute cycle during sleep (but also suggested it continues during the day, even when we are awake).
When we are asleep we move progressively through the sleep stages, and when we are awake we move progressively from a state of alertness into a state of physiological fatigue approximately every 90 mins.
Who came up with the Basic Rest Activity Cycle (BRAC)?
Kleitman (1969).
